1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which feedback-controls the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio, by using an exhaust gas component concentration sensor which has an output characteristic almost proportional to the concentration of a specific component in exhaust gases emitted from the engine.
2. Prior Art
There is conventionally known an air-fuel ratio control system for internal combustion engines, which controls the air-fuel ratio of a mixture supplied to the engine to an air-fuel ratio set in dependence on operating conditions of the engine, by using an exhaust gas component concentration sensor which has an output characteristic almost proportional to the concentration of a specific component in exhaust gases emitted from the engine, and changes the feedback gain according to the set air-fuel ratio (e.g. Japanese Laid-Open Patent Publication (Kokai) No. 4-231636). According to the known control system, when the set air-fuel ratio is a stoichiometric air-fuel ratio, the feedback gain is set to a relatively large value to obtain the maximum purification efficiency of a catalyst arranged in the exhaust system, for purifying noxious components in exhaust gases, while when the set air-fuel ratio is a leaner air-fuel ratio or a richer air-fuel ratio than the stoichiometric air-fuel ratio, the feedback gain is set to a relatively small value.
In the known air-fuel ratio control system, however, the feedback gain is thus set to a relatively large value during execution of the feedback control for controlling the air-fuel ratio to the stoichiometric value. As a result, when a disturbance occurs during execution of the feedback control due to the influence of purging of evaporative fuel generated in the fuel tank, or other factors, the air-fuel ratio of the mixture supplied to the engine can diverge.